How to Generate PDFs with Async and Multithreading

Async and threading are useful when generating high-performance PDFs in C# and VB.NET with IronPDF in batches or for optimized performance.

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First Step:

How to Generate PDFs with Async and Multithreading

Download IronPDF from NuGet to get started with async and multithreading PDF generation
Prepare the HTML contents to be converted
Use the RenderHtmlAsPdfAsync method to convert HTML to PDF asynchronously with IronPDF
Explore using the Parallel.ForEach method for multithreading in PDF processing
Review the performance comparison of different PDF generation techniques

Async Example

IronPDF fully supports Async using its Async Rendering methods such as the RenderHtmlAsPdfAsync method.

:path=/static-assets/pdf/content-code-examples/how-to/async-async.cs

using IronPdf;
using System.Threading.Tasks;

// Instantiate ChromePdfRenderer
ChromePdfRenderer renderer = new ChromePdfRenderer();

string[] htmlStrings = {"<h1>Html 1</h1>", "<h1>Html 2</h1>", "<h1>Html 3</h1>"};

// Create an array to store the tasks for rendering
var renderingTasks = new Task<PdfDocument>[htmlStrings.Length];

for (int i = 0; i < htmlStrings.Length; i++)
{
    int index = i; // Capturing the loop variable
    renderingTasks[i] = Task.Run(async () =>
    {
        // Render HTML to PDF
        return await renderer.RenderHtmlAsPdfAsync(htmlStrings[index]);
    });
}

// Wait for all rendering tasks to complete
// await Task.WhenAll(renderingTasks);

Imports IronPdf
Imports System.Threading.Tasks

' Instantiate ChromePdfRenderer
Private renderer As New ChromePdfRenderer()

Private htmlStrings() As String = {"<h1>Html 1</h1>", "<h1>Html 2</h1>", "<h1>Html 3</h1>"}

' Create an array to store the tasks for rendering
Private renderingTasks = New Task(Of PdfDocument)(htmlStrings.Length - 1){}

For i As Integer = 0 To htmlStrings.Length - 1
	Dim index As Integer = i ' Capturing the loop variable
	renderingTasks(i) = Task.Run(Async Function()
		' Render HTML to PDF
		Return Await renderer.RenderHtmlAsPdfAsync(htmlStrings(index))
	End Function)
Next i

' Wait for all rendering tasks to complete
' await Task.WhenAll(renderingTasks);

VB C#

Multi-Threading Example

IronPDF is thread-safe and supports multithreading when using the IronPdf.ChromePdfRenderer rendering engine.

One limitation is that multithreading is limited on macOS machines.

We have found the Parallel.ForEach pattern very helpful for batch processing PDFs.

:path=/static-assets/pdf/content-code-examples/how-to/async-multi-thread.cs

using IronPdf;
using System.Collections.Concurrent;
using System.Collections.Generic;
using System.Threading.Tasks;

var queue = new List<string>() { "<h1>Html 1</h1>", "<h1>Html 2</h1>", "<h1>Html 3</h1>" };

// Instantiate ChromePdfRenderer
ChromePdfRenderer renderer = new ChromePdfRenderer();

// Create a list to store the rendered PDFs
List<PdfDocument> pdfResults = new List<PdfDocument>();

Parallel.ForEach(queue, html =>
{
    // Render HTML to PDF
    PdfDocument pdf = renderer.RenderHtmlAsPdf(html);

    // You may choose to save the PDF to disk here if needed
    // For this example, we'll store it in the pdfResults list
    lock (pdfResults)
    {
        pdfResults.Add(pdf);
    }
});

Imports IronPdf
Imports System.Collections.Concurrent
Imports System.Collections.Generic
Imports System.Threading.Tasks

Private queue = New List(Of String)() From {"<h1>Html 1</h1>", "<h1>Html 2</h1>", "<h1>Html 3</h1>"}

' Instantiate ChromePdfRenderer
Private renderer As New ChromePdfRenderer()

' Create a list to store the rendered PDFs
Private pdfResults As New List(Of PdfDocument)()

Parallel.ForEach(queue, Sub(html)
	' Render HTML to PDF
	Dim pdf As PdfDocument = renderer.RenderHtmlAsPdf(html)

	' You may choose to save the PDF to disk here if needed
	' For this example, we'll store it in the pdfResults list
	SyncLock pdfResults
		pdfResults.Add(pdf)
	End SyncLock
End Sub)

VB C#

Performance Comparison

Let's perform a comparison. I have additionally added a 5-second delay in rendering with the WaitFor class for simulating complex HTML rendering. Below is the comparison table of the performance using various techniques described above.